Rain

What causes it to rain?
A brief explanation of the four main reasons why rain occurs


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Introduction

For most areas of the world, rain is so common that few people stop to think about it. Why does it rain? Does all rainfall have the same causes?

For those of us living in England, rain is a common subject of discussion and debate. However this essay covers different types of rain from around the world.

Briefly, rain occurs when moist air rises and cools. The cooling causes the moisture to condense and fall as rain. There are four different mechanisms that cause air to rise and each one gives a different type of rain with its own distinct type of cloud and properties. The four types of rain are listed below:


Convection Rain

The Earth's atmosphere varies in temperature with altitude. Generally, air at ground level is warmer. As altitude increases, the air temperature falls. The graph below shows how the temperature of air varies with altitude. The blue line is for air at high pressure; the red line is for air at low pressure. The air pressure is critical as it affects the way that air behaves when the ground is heated.

When the Sun is shining it warms the ground. The warm ground warms the air immediately above it. This produces a bubble of warm air that begins to rise. Warm air rises because warm air is less dense than cold air. The green line in the graph shows, in theory, how a bubble of air at ground level will cool as it rises and expands.

Air temperature

When the atmosphere is at low pressure, the rising air bubble is always warmer than the surrounding air. On the graph, the green line is always above the red line. Under conditions of low pressure, the heated air will continue to rise. As it rises it cools; but it does not cool as quickly as the surrounding air. As it is always warmer than the surrounding air, it continues rising.

Low pressure leads to unstable air conditions. When the ground is heated, the air will rise and keep rising. Eventually, the rising air will cool enough to produce condensation (cloud and rain).

When the atmosphere is at high pressure, the rising air bubble is always cooler than the surrounding air. On the graph, the green line is always below the blue line. Under conditions of high pressure, the heated air will not continue to rise. As it rises it cools; it cools more quickly than the surrounding air. Being cooler than the surrounding air, it will not rise.

High pressure leads to stable air conditions. When the ground is heated, the air will not rise. If air does not rise, rain cannot condense.

Convection rain requires a warm ground and low pressure. It causes short showers interspaced with sunny periods. The rain is caused by rapidly forming Cumulus and Cumulonimbus clouds. It tends to occur in the later part of the day or early evening. Because land warms faster than the sea, this type of rain is far more common over land. Convection causes fluffy Cumulus clouds to appear over land when the nearby sea is cloudless.

It is the main cause of England's April Showers. Moist air arrives over the sea onto warming post-winter ground. This produces unstable conditions leading to showers. The common afternoon showers in tropical regions are also caused by this process. The Sun warms the ground during the day. By afternoon the moist, unstable air is being forced to rise giving rise to huge Cumulonimbus clouds and very heavy rain that arrives rapidly.


Relief Rain

This type of rain is caused by geography.

Air moving along the ground or over the sea, passes hills or mountains. These force the air to rise in order to pass the obstruction. The rising air cools and, if moist, will form mist, clouds or rain on the side of the obstruction where the air is rising. As the air passes over the obstruction, it begins to descend. Descending air warms and will not cause rain.

It is quite common where there is a prevailing wind, for one side of a mountain to be green and lush while the other side is drier. The drier side is called a Rain Shadow.

If the mountains are high enough, the air may not be able to cross them. This gives lush conditions on one side and desert on the other side. The Tibetan Plateau is a high altitude desert because winds from the south drop all their moisture on the southern slopes of the Himalaya and cannot pass over to the other side. The Cloud Forests of Central America are caused by air passing over hilly terrain and condensing its moisture on the leaves of the plants and trees.

Relief rain is characterised by thick cloud with drizzly conditions and can occur at any time, day or night. For obvious reasons it does not occur over flat areas.

Relief Rain


Frontal Rain

Frontal rain is very common in temperate regions. It brings a lot of rain in places like England and New Zealand.

It is caused when two air masses meet.

If a warm air mass meets a cold air mass, a Warm Front is the result. The two air masses will generally not mix. The warm air, being less dense, will gently slide over the cold air. As it rises, it will cool and condense into cloud and rain.

This type of rain takes a long time to arrive. The first clouds are at high altitude (Cirrus clouds) . After a while, the clouds becomes layered and are lower (Stratus). Finally, the clouds turn grey and are very low (Nimbostratus). Rain begins slowly and remains steady for several hours if not days. The barometric pressure will have been dropping slowly as the front approaches. The rain from a Warm Front is not dependent on the presence or absence of the Sun and can occur at any time of the day or night. Warm Fronts can occur over land or sea as they are a phenomenon of the atmosphere.

Warm Front

A Cold Front occurs when a cold air mass meets a warm air mass. Again, the two air masses do not mix. The cold air, being more dense, forces its way under the warm air. The warm air is forced up quickly. This leads to large Cumulonimbus clouds giving heavy showers, often with thunder, lightning and hail. Clear, crisp skies will often follow the rain.

Cold Front


Monsoon Rain

Monsoon rain is a geographical phenomenon. It is caused by cyclic air movements changing with the time of year. To work well, a large land mass needs to be close to a large ocean.

During the dry season air rises over the sea. It moves over the land and descends. Descending air does not produce clouds or rain. The air then moves out to sea where it rises again. This cycle of air requires that the sea is warmer than the land.

At a certain point in the year, the air cycle reverses. The land becomes warmer than the sea. This causes the air to rise over the land, pass out to the sea, descend, then come back over the land. The rising air is now over the land and since it will be moist after passing over the sea, it will condense and rain will fall.

Monsoon rain is seasonal and occurs in certain regions of the Earth, India being one of the best known examples.

Monsoon


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External Weather Links

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The London Meteorological Office
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Guide to Clouds
Descriptions and photographs of the different cloud types.